The present invention relates to a method and an apparatus for changing the size of image data.
It is common for changing the size of image data of an original image to interpolate the image data with the use of interpolation factors determined by the relative distance between the location of pixel data of one field of the original image and the location of pixel data of its size changed image.
A conventional image size changing apparatus employs a known technique of using the fraction of accumulation of an inverse of the size change rate as the interpolation factors (See Japanese Patent Application Laid-open 9-326958). FIG. 15 is a block diagram showing an arrangement of the conventional apparatus.
The conventional apparatus comprises, as shown in FIG. 15, a divider 100 for dividing pixels of the original image and pixels of its size changed image, a horizontal interpolation factor generator circuit 101 for accumulating outputs of the divider 100 at timing of a sampling clock signal and calculating a horizontal interpolation factor from a result of the accumulation, a vertical interpolation factor generator circuit 102 for accumulating outputs of the divider 100 at timing of a horizontal frequency clock signal and calculating a vertical interpolation factor from a result of the accumulation, an address generator circuit 103 responsive to the integer of the accumulation outputs of the horizontal interpolation factor generator circuit 101 and the vertical interpolation factor generator circuit 102 for generating address data used for reading desired data from a memory, the memory 104 for storage of image data of one field of the original image, a pair of inverters 105 and 106 for inverting the interpolation factors from the horizontal 101 and the vertical interpolation factor generator circuit 102 to yield 1-p and 1-q respectively based on 1, a group of multipliers 107, 108, 109, and 10a for multiplying the interpolation factor from the horizontal interpolation factor generator circuit 101 and the interpolation factor from the vertical interpolation factor generator circuit 102 by each other, another group of multipliers 10b, 10c, 10d, and 10e for multiplying the pixel data of one field of the original image received from the memory 104 by the interpolation factors received from the multipliers 107, 108, 109, and 10a respectively, a couple of adders 10f and 10g for summing the outputs of the multipliers 10b and 10c and the outputs of the multipliers 10d and 10e respectively, and an adder 10h. 
The action of the conventional apparatus is explained referring to FIG. 16.
FIG. 16 is a diagram showing the positional relation along the horizontal direction between the location of pixels in one field of the original image and the location of pixels of its size changed image at the size change rate of ⅔. When the size change rate is ⅔, its inverse or 1.5 is accumulated in the horizontal interpolation factor generator circuit 101. As shown in FIG. 16, the size is changed from the reference pixel DS0 or the 0th pixel of one field of the original image. The pixel DD0 in the size changed image corresponding to DS1 is spaced by 1.5 pixels from DS0. It is apparent from this figure that the horizontal interpolation factor p for the pixel DD0 of the size changed image is 0.5. As the interpolation factor for each pixel of the size changed image relative to the pixel of one field of the original image is calculated and used for interpolation of the pixel data, the size changed image will exhibit less foldover effects.
Another conventional image size changing apparatus is also known using as the interpolation factors a sum of the fraction of accumulation of an inverse of the size change rate and the offset for the size change rate (Japanese Patent Application Laid-open 11-25265). FIG. 17 is a block diagram showing an arrangement of the conventional apparatus.
The conventional apparatus shown in. FIG. 17 comprises a magnification setting 203 for setting a desired level of the size change rate, an interpolation factor generator 201 for calculating the interpolation factors from the size change rate, an offset circuit 202 for calculating the interpolation factors for calculating the interpolation factors from the size change rate, an interpolator 200 for interpolating an input image signal SI with the interpolation factors received from the interpolation factor generator 201, a field memory 205 for storage of interpolated image data received from the interpolator 200, and a read/write controller 204 for controlling the reading and writing of the data on the field memory 205. The image data retrieved from the field memory 205 by the action of the read/write controller 204 is transferred to the outside as an output image signal SO.
The action of the conventional apparatus is now explained referring to FIG. 18.
FIG. 18 is a diagram showing the positional relationship along the horizontal direction between the location of pixels of one field of the original image and the location of pixels of its size changed image at the size change rate of ⅔. The calculation of interpolation factors in the conventional apparatus is substantially similar to that described previously with FIGS. 15 and 16. This apparatus is however differentiated by the fact that the fraction of the accumulation of an inverse of the size change rate is added with the offset corresponding to the size change rate. Assuming that the size change rate is a=d/s, the offset b is b=1/(2xc3x97d) As the interpolation factors used for interpolation are determined from the distance data averaged between pixels, the size changed image will exhibit less foldover effects.
However, such conventional apparatuses have the following drawbacks.
The first drawback resides in the conventional apparatus shown in FIG. 15 where the interpolation factors are calculated from a difference between the distance between pixels in one field of the original image and the distance between pixels in the sized changed image. Because a difference between the two distances is not always present, the interpolation between pixels may sometimes be disabled. Accordingly, the data ratio between pixels in the size changed image will hardly be uniform thus causing an undesired artifact in the image.
While the conventional apparatus shown in FIG. 17 can successfully avoid the data ratio between pixels in the size changed image from being varied to cause degradation of the image quality, the second drawback resides in both the conventional apparatuses shown in FIGS. 15 and 17 where the interpolation factors are calculated from distance data between the location of a pixel in one field of the original image and the location of its corresponding pixel in the size changed image, hence failing match a vertical ratio between a visual area determined by pixel data of the original image and a visual area determined by pixel data of the size changed image. This result in an imperfect action of the interpolation, causing an undesired artifact in the size changed image.
It is an object of the present invention to provide a method and an apparatus for changing the size of image while eliminating the above-described drawbacks.
Other objects of the present invention will become clear as the description proceeds.
An image size changing method of the present invention for changing the size of an original image according to a size change rate and releasing a resultant size changed image is provided comprising the steps of: generating interpolation factors from the size change rate; and interpolating the original image with the interpolation factors to develop the size changed image, wherein the interpolation factor generating step is arranged for calculating the interpolation factors having a ratio between a visual area determined by pixel data of the original image and a visual area determined by pixel data of its size changed image.
An image size changing apparatus of the present invention for changing the size of an original image according to a size change rate and releasing a resultant size changed image is provided comprising: an interpolation factor generating means for generating interpolation factors from the size change rate; and an interpolating means for interpolating the original image with the interpolation factors to develop the size changed image, wherein the interpolation factor generating means is arranged for calculating the interpolation factors having a ratio between a visual area determined by pixel data of the original image and a visual area determined by pixel data of its size changed image.